In recent years, inkjet recording that uses water-soluble inks has been significantly developed and increasingly widely used. Inkjet recording creates color images by jetting fine droplets of inks based on various operation principles to allow them to impact on recording sheets such as paper. Inkjet recording is fast and quiet, facilitates multicolor printing, is versatile in terms of recordable patterns, and is free from the need of development or fixation. Thus, inkjet recording is employed by printing machines for various applications. Inkjet recording also creates images that are comparable to prints produced by offset printing or by a color photographic technique. Additionally, a small number of copies can be printed by inkjet recording more inexpensively than by offset printing or a color photographic technique. For these reasons, inkjet printing is widely employed in the commercial printing field.
Industrial inkjet printing machines for commercial printing, in particular, industrial inkjet printing machines of a web press system, are used in on-demand printing applications such as printing of addresses, customer information, numbers, and bar codes. In such an on-demand printing, fixed information is often printed by offset printing and then variable information is printed by inkjet printing.
Industrial inkjet printing machines of a web press system that have a printing speed of 60 m/min or higher, and a still higher speed exceeding 120 m/min have recently been developed. Hence, there is a growing need for coated printing paper having ink absorption properties suitable for printing at such high speeds using an industrial inkjet printing machine.
Additionally, to meet the demand for high-definition and high-quality commercial printing, there is also a need for coated printing paper that is usable in industrial inkjet printing machines and that has similar textures to those of coated printing paper such as general-purpose matte coated woodfree (CWF) paper and gloss coated woodfree (CWF) paper.
The inks used in industrial inkjet printing machines include water-based dye inks and water-based pigment inks. These inks require different properties of coated printing paper.
The water-based dye inks need coated printing paper that achieves improved color densities and water resistance of printed images on the paper. More specifically, the water-based dye inks need coated printing paper that enables high color densities and vivid color tone, and that prevents the inks from bleeding when the paper is left, after printing, under humid conditions or when printed sections are exposed to water for some reason.
The water-based pigment inks need coated printing paper that enables improved scratch or abrasion resistance of printed images on the paper. More specifically, the water-based pigment inks need coated printing paper that prevents the inks from coming off from printed sections when rubbed with something, after being printed and dried, so that printed matter is free from smears.
The water-based pigment inks also need coated printing paper that can prevent or reduce uneven printing. Uneven printing is a phenomenon in which printing paper exhibits non-uniform saturation of inks that are fixed in the final printed image, after the inks are dried, due to non-uniform ink absorption by the printing paper during high speed printing. Because inks for industrial inkjet printing machines have a lower amount of colorants, uneven printing is conspicuous in the printed image compared with offset printing. Thus, the water-based pigment inks need coated printing paper that eliminates or reduces uneven printing when printed at a high speed.
The coated printing paper is required to be printable at a high speed with desirable absorption properties, regardless of whether the inks are water-based dye inks or water-based pigment inks.
High gloss coated paper that is composed of two or more coating layers and usable in offset printing and inkjet printing has been described. Specifically, an inkjet recording paper having two coating layers, a lower coating layer that contains kaolin, precipitated calcium carbonate, and an aqueous adhesive, and an upper coating layer that contains silica sol and/or hydrated alumina, a dye fixer, and an alkali metal salt and/or an alkaline-earth metal salt is known (see, for example, Patent Document 1). Another known example is an inkjet recording paper having two coating layers, a lower coating layer that contains kaolin, precipitated calcium carbonate, and an aqueous adhesive, and an upper coating layer that contains a pigment that has a mean particle size of 0.01-1 μm and is selected from silica, alumina, and hydrated alumina, and a water-soluble resin binder (see, for example, Patent Document 2). Still another known example is an inkjet recording paper having two coating layers, an undercoating layer that contains an alkaline-earth metal salt and an organic pigment, and a top coating layer that contains an inorganic fine particle with a primary particle size of 100 nm or less and a secondary particle size of 400 nm or less, in which the coating color for the top coating layer has a pH of 5.0 or less (see, for example, Patent Document 3). Yet another known example is a pigment coated paper for printing having at least two layers including a pigment coating inner layer that contains precipitated calcium carbonate and a styrene-butadiene copolymer latex, and an outermost pigment coating layer including a coating layer that contains precipitated calcium carbonate with a mean particle size (minor axis) of 0.8 μm or less and a styrene-butadiene copolymer latex, on which a water-soluble multivalent metal salt is applied (see, for example, Patent Document 4).